AWS Elastic Beanstalk "is leader instance" check for use with cron, etc (nodejs, async/await).

aws_ebs_leader.js

// based on https://gist.github.com/tony-gutierrez/de5b304fd042f6140eb61a31d0ff92d5
async function isLeader() {
    try {
        // get APIs
        var elasticbeanstalk = new AWS.ElasticBeanstalk();
        var ec2 = new AWS.EC2();
        var metadata = new AWS.MetadataService();
        // get ec2 instance id
        const request = new Promise( (resolve, reject) => { // MetadataService.request has no support for promise, make one ourselves
            metadata.request('/latest/meta-data/instance-id', (err, InstanceId) => {
              if (err) {return reject(err);}
              return resolve(InstanceId)
            });
        });
        const currentInstanceId = await request;
        console.log('currentInstanceId', currentInstanceId);
        // get env id
        var params = {
            Filters: [
                {
                    Name: 'resource-id',
                    Values: [currentInstanceId]
                }
            ]
        };
        const tagData = await ec2.describeTags(params).promise(); // needs ec2:DescribeTags permission
        console.log('tagData', tagData);
        const envIdTag = tagData.Tags.find(t => t.Key === 'elasticbeanstalk:environment-id');
        if (envIdTag === null) {
            throw Error('Failed to find the value of "elasticbeanstalk:environment-id" tag.');
        }
        // get ebs instances for env
        // needs elasticbeanstalk:DescribeEnvironmentResources, autoscaling:DescribeAutoScalingGroups
        const envData = await elasticbeanstalk.describeEnvironmentResources({ EnvironmentId: envIdTag.Value }).promise();
        console.log('envData', envData);
        console.log('instances', envData.EnvironmentResources.Instances);
        // see if we are first
        return currentInstanceId === envData.EnvironmentResources.Instances[0].Id;
    } catch(e) {
        console.error(e);
        return false;
    }
}

// https://stackoverflow.com/questions/20904787/get-current-environment-name
function isCloud() {
    try {
        var contents = fs.readFileSync('/sys/hypervisor/uuid', 'utf8');
        return contents.substr(0,3) === 'ec2';
    } catch(e) {
        // console.error(e); // expected to get: Error: ENOENT: no such file or directory, open '/sys/hypervisor/uuid'
        return false;
    }
}